1. Field of the Invention
The invention relates to a strap driving device as it is used in particular in looping machines.
2. Background Art
In particular when used in looping machines, a looping strap driving device of the generic type should offer the possibility of prompt activation and deactivation. This provides for the prerequisite that the switch-over from insertion to withdrawal of the looping strap for the latter to fit around the product stack to be looped can be carried out in a short time.
Fundamentally, a plurality of solutions in terms of constructional details is known for strap driving devices. For instance, reference can be made to EP 0 005 508 A2 or DE 32 49 559 C2. These known variants are based on a common principle as regards the arrangement of the drive rollers. These are cylindrical bodies of revolution which are oriented such that a gap is available between their surface areas, through which the strap passes in its flattened position. The drive rollers act on the two main surfaces of the looping strap through frictional engagement by their surface areas so that the looping strap is transported by rotation of the drive rollers. In this case it is basically possible to drive both rollers synchronously and in opposite directions as in the case of the strap driving device according to EP 0 005 508 A2. By alternative it is also possible that only one of the two rollers is driven actively. The second roller functions as a co-rotating pressing roller as is the case with the looping strap advancing and tensioning device according to DE 32 49 559 C2.
The strap driving device according to DE 195 36 964 A1 is based on a deviating principle. In this case drive rollers are used for the transmission of a high driving power, the surface areas of which are provided with a continuous key groove. The looping strap frictionally engages with this key groove by its respectively allocated lateral edges.
DE 32 49 559 C2 mentioned above teaches a strap driving device having an advance and withdrawal wheel driven in rotation and disposed stationarily on the machine; a tensioning wheel mounted to be driven in rotation on a pivot lever; and a co-rotating pressing roller rotatably mounted on a rocker. Triggering the drives of the advance and withdrawal wheel on the one hand and the tensioning wheel on the other and corresponding spring-loading of the pivot lever and the rocker during the insertion of the looping strap as well as during its withdrawal and tightening helps attain a comparatively complex way of operation. Trouble-free functioning then depends on an accurate adjustment of various constructional elements such as spring-loads, limit stops etc. In this regard, the design and adjustment of this strap driving device are complicated. Furthermore, an advance and withdrawal wheel as well as a tensioning wheel are provided for strap driving, both of them cooperating with a single co-rotating pressing roll. In this regard, the advance and withdrawal wheel on the one hand and the tensioning wheel on the other hand are disposed at different circumferential positions of the pressing roll so that the way of the looping strap through the strap driving device takes a strongly curved, S-shaped course. This complicates the threading of the strap considerably and experience has shown that this gives rise to troubles in the passage of the strap.